1. Field of the Invention
The present invention relates to a method for adjusting a system for providing hearing assistance to a user; it also relates to a corresponding system. In particular, the invention relates to a system comprising a microphone arrangement for capturing audio signals, a transmission unit for transmitting the audio signals via a wireless audio link from the transmission unit to a receiver unit, and means worn at or in the user's ear for stimulating the hearing of the user according to the audio signals received by the receiver unit.
2. Description of Related Art
Usually in such systems the wireless audio link is an FM radio link. According to a typical application of such wireless audio systems the receiver unit is connected to or integrated into a hearing instrument, such as a hearing aid, with the transmitted audio signals being mixed with audio signals captured by the microphone of the hearing instrument prior to being reproduced by the output transducer of the hearing instrument. The benefit of such systems is that the microphone of the hearing instrument can be supplemented or replaced by a remote microphone which produces audio signals which are transmitted wirelessly to the FM receiver and thus to the hearing instrument. In particular, FM systems have been standard equipment for children with hearing loss in educational settings for many years. Their merit lies in the fact that a microphone placed a few inches from the mouth of a person speaking receives speech at a much higher level than one placed several feet away. This increase in speech level corresponds to an increase in signal-to-noise ratio (SNR) due to the direct wireless connection to the listener's amplification system. The resulting improvements of signal level and SNR in the listener's ear are recognized as the primary benefits of FM radio systems, as hearing-impaired individuals are at a significant disadvantage when processing signals with a poor acoustical SNR.
Most FM systems in use today provide two or three different operating modes. The choices are to get the sound from: (1) the hearing instrument microphone alone, (2) the FM microphone alone, or (3) a combination of FM and hearing instrument microphones together.
Usually, most of the time the FM system is used in mode (3), i.e. the FM plus hearing instrument combination (often labeled “FM+M” or “FM+ENV” mode). This operating mode allows the listener to perceive the speaker's voice from the remote microphone with a good SNR while the integrated hearing instrument microphone allows to listener to also hear environmental sounds. This allows the user/listener to hear and monitor his own voice, as well as voices of other people or environmental noise, as long as the loudness balance between the FM signal and the signal coming from the hearing instrument microphone is properly adjusted. The so-called “FM advantage” measures the relative loudness of signals when both the FM signal and the hearing instrument microphone are active at the same time. As defined by the ASHA (American Speech-Language-Hearing Association 2002), FM advantage compares the levels of the FM signal and the local microphone signal when the speaker and the user of an FM system are spaced by a distance of two meters. In this example, the voice of the speaker will travel 30 cm to the input of the FM microphone at a level of approximately 80 dB-SPL, whereas only about 65 dB-SPL will remain of this original signal after traveling the 2 m distance to the microphone in the hearing instrument. The ASHA guidelines recommend that the FM signal should have a level 10 dB higher than the level of the hearing instrument's microphone signal at the output of the user's hearing instrument.
When following the ASHA guidelines (or any similar recommendation), the relative gain, i.e. the ratio of the gain applied to the audio signals produced by the FM microphone and the gain applied to the audio signals produced by the hearing instrument microphone, has to be set to a fixed value in order to achieve e.g. the recommended FM advantage of 10 dB under the above-mentioned specific conditions. Accordingly,—depending on the type of hearing instrument used—the audio output of the FM receiver has been adjusted in such a way that the desired FM advantage is either fixed or programmable by a professional, so that during use of the system the FM advantage—and hence the gain ratio—is constant in the FM+M mode of the FM receiver.
CA 2422449 A1 relates to an example of such an FM receiver which not only receives audio signals from a remote microphone transmitter but in addition may communicate with remote devices such as a remote control or a programming unit via wireless link for data transmission.
EP 1 638 367 A2 relates to another example of an FM receiver for receiving audio signals from a remote microphone transmitter, wherein the FM receiver upon receipt of a polling signal from the remote microphone transmitter is capable of transmitting status information regarding the FM receiver to the remote microphone transmitter.
WO 97/21325 A1 relates to a hearing system comprising a remote unit with a microphone and an FM transmitter and an FM receiver connected to a hearing aid equipped with a microphone. The hearing aid can be operated in three modes, i.e. “hearing aid only”, “FM only” or “FM+M”. In the FM+M mode the maximum loudness of the hearing aid microphone audio signal is reduced by a fixed value between 1 and 10 dB below the maximum loudness of the FM microphone audio signal, for example by 4 dB. Both the FM microphone and the hearing aid microphone may be provided with an automatic gain control (AGC) unit.
WO 02/30153 A1 relates to a hearing system comprising an FM receiver connected to a digital hearing aid, with the FM receiver comprising a digital output interface in order to increase the flexibility in signal treatment compared to the usual audio input parallel to the hearing aid microphone, whereby the signal level can easily be individually adjusted to fit the microphone input and, if needed, different frequency characteristics can be applied. However, is not mentioned how such input adjustment can be done.
Contemporary digital hearing aids are capable of permanently performing a classification of the present auditory scene captured by the hearing aid microphones in order to select the hearing aid operation mode which is most appropriate for the determined present auditory scene. Examples for such hearing aids with auditory scene analyses can be found in US2002/0037087, US2002/0090098, CA 2439427 A1 and US2002/0150264.
Usually FM or inductive receivers are equipped with a squelch function by which the audio signal in the receiver is muted if the level of the demodulated audio signal is too low in order to avoid user's perception of excessive noise due a too low sound pressure level at the remote microphone or due to a large distance between the transmission unit and the receiver unit exceeding the reach of the FM link, see for example U.S. Pat. No. 5,734,976 and EP 1 619 926 A1.
As already mentioned above, usually the FM advantage is set to a value of about 10 dB, which value is a compromise taking into account a medium surrounding noise level and a good intelligibility of both the FM audio signal and the voice of the neighbours. Further, this value is based on a medium sensitivity of the hearing aid audio input and on a specific microphone impedance of the hearing aid microphone. Variations of the audio input sensitivity of different hearing aids due to microphone impedance and/or sensitivity variations will have a direct impact on the desired FM advantage of 10 dB, i.e. they will cause a deviation from this desired value, resulting in a decreasing comprehension and listening comfort. Measurements have shown audio input sensitivity variations of up to ±6 dB between the main hearing aid models present in the market. This implies that in practice the FM advantage will vary between 4 dB and 16 dB, depending on the hearing aid model connected to the FM receiver, instead of the desired value of 10 dB. In addition to that, tolerances of the FM transmitter and FM receiver gain are also added to the total FM advantage variation. Further, the desired FM advantage of 10 dB is a recommendation only and may not be optimum in any case or situation. In specific cases, the individual user's perception may require another value of the FM advantage than 10 dB.
It is an object of the invention to provide for a method for adjusting a system for providing hearing assistance to a user, wherein a remote microphone arrangement coupled by a wireless audio link to a receiver unit worn by the user is used and wherein perception of the transmitted audio signals should be optimized for the specific user, independently of the hearing instrument model and the FM system parameter variations and tolerances. It is a further object to provide for a corresponding system.